High-power fiber lasers have received a wide attention in the past ten years. Such lasers with several kilowatts (kWs) or several tens of kWs have been used as commercially available products in industries. In comparison with solid-state lasers, fiber lasers have a unique feature of a superb beam quality at high power due to an all-fiber configuration. That is, all the optical components used in the fiber lasers are of an optical fiber type, connected using fusion splices without air interfaces between any two of the optical components in connection. The optical components include multiple diode laser pumps with multiple optical fiber pigtails, an amplification optical fiber with two fiber Bragg gratings, a transmission fiber spliced to the amplification optical fiber, and an optical fiber combiner with multiple input optical fibers to splice to the multiple optical fiber pigtails of the multiple diode laser pumps and with an output optical fiber to splice to the amplification optical fiber. The amplification optical fiber, doped with a rare earth element such as erbium (Er) or ytterbium (Yb) as a gain medium, provides for a beneficial geometry and a large surface to volume ratio, thus allowing for extraordinary heat dispersion and reducing thermal lensing effect when compared to rod type solid state lasers. The amplification optical fiber with the gain medium receives and absorbs optical energy from the multiple diode laser pumps through the optical fiber combiner and creates a coherent laser light via a resonator built by using the two fiber Bragg gratings at two ends of the amplification optical fiber. Such multimode fiber lasers in the 2- to 6-kW regime are ideal for cutting and welding, and particularly in the area of materials processing and laser machining as a reliable replacement for bulky diode pumped solid-state lasers and CO2 lasers. It has been shown that lengthening the amplification optical fiber can inherently increase power of the fiber lasers without a limit. However, double clad optical fibers (DCOFs) used in both the output optical fiber of the optical fiber combiner and the amplification optical fiber are surrounded by a polymer coating with a limited tolerance to heat. In other words, the maximum thermal load provided by the coating dictates the maximum output power that the fiber laser can attain.
Not similar to optical fibers used in optical communications, where the coatings outside the optical fibers simply play a role of mechanical protection, the polymer coatings used in DCOFs perform both mechanical and optical functions. DCOFs use dual acrylate coatings, with a first low refractive index polymer coating in contact with the glass core, and with a durable second coating to protect the first relatively soft low refractive index coating. In other words, the second coating mechanically protects the low refractive index coating from mechanical chips, cuts, or scratches which may result in optical energy to leak out from the fiber, possibly creating localized hot spots or catastrophic burns at high pump powers. DCOFs with the dual acrylate coating can pass the stringent reliability test specified by Telcodia GR-20 standard used in the telecom industry. Without doubt, DCOFs with the dual acrylate coating have a high tensile strength of greater than 700 kilo-pounds per square inch (kpsi) and an exceptional stress corrosion resistance. However, according to the GR-20 standard, after exposing DCOFs to an environment of 85° C. and 85% relative humidity (RH) for 720 hours, it shows an excess loss for laser output power due to possible degradation of the low refractive index coating in exposure to temperature and humidity. It is noted that the 85° C./85% condition not only affects the optical reliability of the low refractive index coating but also causes OH ingression into the glass core of the optical fiber, increasing attenuation of the glass core. For example, the attenuation in the typical pump wavelength range is well below a negligible 0.01 dB/m. After exposure the optical fiber to temperature and humidity, either wavelength-dependent or independent attenuation increases. The attenuation, in general, is associated with OH ingression in the silica, glass defects formed due to moisture ingression, and light scattered by the low refractive index polymer. That is, during the 85° C./85% RH test, moisture not only degrades the low-index polymer but also penetrates the glass cladding, resulting in the excess fiber loss.
An N×1 tapered fiber bundles (TFBs) or optical fiber combiner is used to combine multiple (“N”) input multimode fiber pigtails from pump diodes into a single output fiber. The “N” satisfies the brightness conservation theorem, and the maximum “N” is 6, 13, 17, 24, 53, 63, 136, etc., depending on various combinations of various diameter and numerical aperture (NA) of the input optical fibers and the output optical fiber. In practice, the N is chosen to be far smaller than the maximum numbers specified above to provide some margin. The optical fiber combiner is typically fabricated in a process similar to fused fiber couplers by bundling in parallel N multimode optical fibers that have been stripped of their polymer coatings. The fibers are then fused and tapered by heating with a flame such as electric arc, oxyhydrogen flame, or a CO2 laser beam. The fused and tapered section is then cleaved in the middle and spliced to the single output fiber. The use of optical fiber combiners to combine multiple laser diode pumps into one fiber is essential for pumping the high-power fiber lasers. For a 7×1 combiner, each of seven input optical fibers with 200-μm diameter and 0.22 NA receives, for example, 200 W from each diode laser pump. Seven such laser pumps are combined into a single 400 μm double-clad fiber with 0.46 NA. This configuration gives a pumping module composed of active and passive components, delivering 1.4 kWs power for a fiber laser, based on the commercially available 200-W laser diode pumps. For more examples, with a Yb-doped fiber of 400 μm and 0.46 NA, a common optical fiber combiner coupling six 200 μm 0.22 NA pump delivery fibers each with a pump power of 500 W provides a total power greater than 3 kWs. Using a 19×1 optical fiber combiner and greater than 100-W pump power delivered in each 105-μm input optical fiber, a total of about 2-kW pump power can be achieved.
The optical fiber combiners can also be used in optical fiber amplifiers to combine pump and signal light that is confined to the core of the double-clad fiber. In this housing, the fiber in the center of the optical fiber combiner is replaced by a fiber with a core carrying an amplifier seed. This is commonly referred to as an (N+1)×1 combiner, which is critical for the optical fiber amplifiers. As an example, a (6+1)×1 combiner accommodating six pump fibers and one signal fiber can be used for a 1-kW co-pumped optical fiber amplifier, based on six pump diodes each delivering, for example, 250 W of pump power for a total pump power of 1.5 kWs. No matter whether 7×1 or (6+1)×1, the optical fiber combiner needs to be thermally managed to maintain its reliability. Specifically, the residual pump power, ASE power, and unwanted signal power trapped in the cladding of a double-clad fiber in the fiber laser or the optical fiber amplifier need to be removed to avoid potential damage to components downstream. The residual pump power can be in the hundreds of watts in kW fiber lasers and the ASE can be in the range of many watts, typically much higher in the optical fiber amplifier. The unwanted energy launching into the cladding of a double-clad fiber creates localized hot spots or catastrophic burns at high pump powers. The most efficient way to remove the cladding light is to strip the low-index fluoroacrylic coating off a length of the fiber and re-coat it with a high-index coating so that high-NA cladding light can be stripped.
As mentioned above, high-power optical fiber combiners are critical for highly reliable high-power fiber lasers. In the high-power fiber lasers, an integrated water-cooled package has been proposed, in which an N×1 optical fiber combiner is completely immersed in the circulating water for efficient cooling. In this housing, however, OH ingression in the silica and glass defects generated from moisture ingression can reduce the reliability of such N×1 optical fiber combiner. It is, therefore, the purpose of this patent application to disclose a thermal dispersion scheme that can be used in packaging the optical fiber combiner without immersing the optical fiber combiner in cooling water while effectively removing heat from so called localized hot spots. In this case, the N×1 optical fiber combiner remains its reliability by not exposing the N×1 optical fiber combiner to water.